NotDoor is a newly identified backdoor malware attributed to APT28, a Russian intelligence-linked advanced persistent threat group known for targeting government, military, and strategic sectors. This backdoor is implemented as a VBA macro embedded within Microsoft Outlook, which monitors incoming emails for specific trigger words to activate its malicious capabilities. Once triggered, NotDoor enables a range of offensive actions including data exfiltration, file uploads to the victim machine, and arbitrary command execution. The malware leverages DLL side-loading via the legitimate Microsoft OneDrive.exe process to execute its payload, a technique that helps it evade detection by blending with trusted system processes. Persistence is established through modifications to Windows registry keys, ensuring the malware remains active across system reboots. NotDoor also employs multiple obfuscation techniques and a custom string encoding method to hinder analysis and detection. The malware's capabilities include executing commands remotely, exfiltrating sensitive files, and uploading files onto compromised systems. This evolution in APT28's toolkit demonstrates their ongoing efforts to bypass modern defense mechanisms and maintain stealthy access to targeted networks. The threat is particularly concerning for NATO member countries, as APT28 has historically focused on espionage and disruption campaigns against Western military and governmental organizations. The campaign does not currently have known exploits in the wild but represents a medium-severity threat due to its sophisticated evasion and persistence techniques combined with its potential impact on confidentiality and integrity of sensitive information.

For European organizations, especially those within NATO member states, NotDoor poses a significant espionage and operational risk. The malware's ability to monitor Outlook emails and selectively activate on trigger words allows targeted infiltration of high-value individuals and departments, such as defense, foreign affairs, and critical infrastructure operators. Data exfiltration capabilities threaten the confidentiality of sensitive government communications and classified information. The ability to upload and execute arbitrary commands can lead to lateral movement within networks, potentially compromising additional systems and enabling further espionage or sabotage. The use of DLL side-loading via a trusted Microsoft executable complicates detection, increasing the likelihood of prolonged undetected presence. This could result in significant operational disruption, loss of intellectual property, and exposure of strategic plans. The persistence mechanism ensures that even after system restarts, the threat remains active, increasing the difficulty of eradication. Given the geopolitical tensions involving Russia and NATO countries, this malware could be part of broader cyber espionage campaigns aimed at undermining European security and defense capabilities.

To mitigate the NotDoor threat, European organizations should implement a multi-layered defense strategy tailored to the specific tactics used by this malware. First, restrict or disable VBA macros in Outlook unless explicitly required and signed by trusted publishers. Employ application whitelisting to prevent unauthorized DLLs from loading via legitimate executables like OneDrive.exe. Monitor and audit registry key changes related to persistence mechanisms, particularly those associated with user startup and system services. Deploy advanced endpoint detection and response (EDR) solutions capable of detecting DLL side-loading and anomalous process behaviors. Implement network monitoring to detect unusual outbound traffic patterns indicative of data exfiltration. Use threat intelligence feeds to update detection signatures with the provided file hashes and indicators of compromise. Conduct regular phishing awareness training to reduce the risk of initial infection via email. Finally, enforce strict access controls and network segmentation to limit lateral movement if a system is compromised.